The invention discloses a preparation method of benzoxazine resin. The preparation method comprises the following steps: S1, adding
activated carbon particles into an alkali solution for soaking to obtain an
activated carbon catalyst adsorbing alkali; S2, adding a
solvent toluene, phenolic compounds,
aldehyde and amine compounds into a reaction still in sequence, keeping the temperature of the reaction still at 40 DEG C, stirring and adding the
activated carbon catalyst; S3, raising the temperature to 70 DEG C and then stopping heating; continuously raising the temperature of reaction liquid by
waste heat; when the temperature reaches 85 DEG C, carrying out
reflux reaction on the reaction liquid for 2 to 6 hours; S4, replacing the reaction still with an atmospheric
distillation device, raising the temperature to 110 DEG C for distilling, returning the
toluene in distillate to the inner part of the reaction still, distilling until no water is drained from the reaction still, cooling anddischarging; S5, filtering the discharged material, and obtaining filtrate, namely a
toluene solution containing benzoxazine; washing and
drying filter residues and recycling the activated carbon catalyst. According to the preparation method of the benzoxazine resin, disclosed by the invention, the catalyst alkali is immobilized in activated carbon, so that the catalyst can be recycled and a washing operation step in an existing preparation method is omitted.